Multipath effect suppression method for 2.4 GHz radio frequency identification

Abstract

The invention discloses a multipath effect suppression method for 2.4 GHz radio frequency identification. The multipath effect suppression method comprises a transmitting end processing flow and a receiving end processing flow. A transmitting end carries out pulse code modulation on an analog signal to obtain a digital signal, blocks the digital signal after channel coding, and distributes the digital signal to subcarriers through quadrature amplitude modulation; and generating a time domain OFDM symbol through IFFT (Inverse Fast Fourier Transform), adding a cyclic prefix and a synchronization head, converting into an analog baseband signal, and performing up-conversion, filtering, amplification and radiation. The receiving end amplifies the signal, down-converts the signal to a baseband, filters the signal and converts the signal into a digital signal; the method comprises the following steps of: synchronizing header frame alignment, recovering a coding bit stream through FFT (Fast Fourier Transform) and QAM (Quadrature Amplitude Modulation) demodulation, carrying out CRC (Cyclic Redundancy Check) after decoding and merging, returning ACK (Acknowledgement Character) if According to the method, the multipath effect can be effectively inhibited, and the integrity of received data is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of radio frequency identification, and particularly to a method for suppressing multipath effects. Background Art

[0002] RFID (Radio Frequency Identification) is an important branch of automatic identification technology. It realizes non-contact two-way data communication through wireless radio frequency, and completes target identification and data exchange by wirelessly reading and writing recording media such as electronic tags (or radio frequency cards). Among them, 2.4GHz radio frequency identification chips are widely used in the fields of logistics management, inventory tracking, personnel identification, etc. due to their high transmission rate, long communication distance, low power consumption characteristics and strong anti-interference ability. Such chips support multi-tag concurrent reading technology, and can establish communication links with multiple tags simultaneously, significantly improving the system identification efficiency and scalability.

[0003] In practical applications, 2.4GHz band chips face the technical challenge of multipath effects. This effect refers to the physical phenomena such as reflection and diffraction of radio frequency signals during propagation, forming multiple different propagation paths. When the signals of each path reach the receiving end, superposition interference occurs due to phase and amplitude differences, resulting in distortion or fading of the received signal.

[0004] Traditional solutions, such as optimizing antenna design, improving environmental layout, and improving signal processing algorithms, have significant limitations: 1. Antenna optimization may lead to an increase in hardware cost and an increase in module volume; 2. Environmental transformation is restricted by application scenarios (such as industrial warehousing, complex building environments), and the implementation difficulty is high; 3. Algorithm improvement may introduce computational complexity, reduce real-time performance or sacrifice signal-to-noise ratio. Summary of the Invention

[0005] Object of the Invention: Aiming at the above-mentioned prior art, a method for suppressing multipath effects in 2.4GHz radio frequency identification is proposed to improve the integrity of received data.

[0006] Technical Solution: A method for suppressing multipath effects in 2.4GHz radio frequency identification includes a transmitter processing flow and a receiver processing flow; The transmitter processing flow specifically includes: Step 1: Sample, quantize, and encode the input analog signal in sequence to complete pulse code modulation, convert the analog signal into a digital signal; after channel coding the digital bit stream, divide it into blocks according to the number of subcarriers N, and fill in zeros if there is a shortage; then perform quadrature amplitude modulation on each block of data, map it to complex plane modulation symbols and allocate them to N orthogonal subcarriers; Step 2: Perform N-point IFFT transformation on the N subcarrier frequency-domain symbols to generate time-domain OFDM symbols, extract the last N samples of the symbols and splice them to the front end to form a complete OFDM symbol, and add a 4-byte synchronization header at the beginning of the frame; cp The last N samples are used as the cyclic prefix and spliced to the front end to form a complete OFDM symbol, and a 4-byte synchronization header is added at the beginning of the frame; Step 3: Splice the OFDM symbol with the cyclic prefix and the synchronization header into a serial data stream. After parallel-to-serial processing, it is converted into an analog baseband signal by a digital-to-analog converter; Step 4: Mix the analog baseband signal with a 2.4 GHz local oscillator for up-conversion, filter out harmonics through a band-pass filter, amplify it to the rated power using a power amplifier, and radiate it through an antenna; The receiving-end processing flow specifically includes: Step 1: The received signal is amplified by a low-noise amplifier, mixed with the local oscillator for down-conversion to the baseband, and after anti-aliasing filtering, it is converted into a digital signal by an analog-to-digital converter at the sampling rate of the transmitting end; Step 2: Achieve frame alignment by detecting the synchronization header, split the OFDM symbol and discard the cyclic prefix, perform N-point FFT transformation on the valid symbol, restore the frequency-domain symbol and separate the data of each subcarrier; Step 3: Perform QAM demodulation on each subcarrier symbol that matches the transmitting end to restore the encoded bit stream. After decoding to correct the error codes, merge the data in the block sequence and remove the zero-padding bits to restore the original sequence; Step 4: Perform CRC check on the restored data. If the check passes, return ACK; if it fails, return NACK to trigger the transmitting end to retransmit.

[0007] Further, in Step 1 of the transmitting-end processing flow, the number of subcarriers N is determined by the available bandwidth of the system.

[0008] Further, in Step 2 of the transmitting-end processing flow, the cyclic prefix length N cp satisfies: N cp ≥ maximum multipath delay / sampling period.

[0009] Further, in Step 4 of the transmitting-end processing flow, a class AB power amplifier is used, and the linearity index ACLR ≤ -30 dBc.

[0010] Further, in Step 1 of the transmitting-end processing flow, channel coding is implemented using a convolutional code encoder; in Step 3 of the receiving-end processing flow, Viterbi decoding is used to complete channel decoding.

[0011] Further, in Step 4 of the receiving-end processing flow, after returning ACK when the check passes, it enters the low-power standby mode.

[0012] Beneficial effects: In a 2.4GHz radio frequency identification system, the multipath effect causes the transmitted signal to reach the receiving end through multiple physical paths (such as reflection and diffraction by obstacles). The signals on each path are superimposed and interfered due to phase and amplitude differences, resulting in distortion of the received signal and deterioration of data integrity. The present invention proposes a communication mechanism based on orthogonal frequency division multiplexing (OFDM), which divides the broadband signal into multiple low-speed orthogonal subcarriers for transmission. By extending the symbol period to be much larger than the typical multipath delay range, the damage of multipath to the symbol boundary is reduced. A cyclic prefix (CP) is introduced in the frame structure design, which can limit the multipath trailing part within the redundant interval, realize the conversion from linear convolution to cyclic convolution, and keep the orthogonality of each subcarrier undamaged, further improving the anti-interference ability. At the same time, convolutional coding is adopted at the transmitting end to increase the data redundancy, and the Viterbi algorithm is used at the receiving end to complete error correction. Under the guarantee of the symbol synchronization mechanism, the effective carrier symbols are accurately extracted, improving the channel robustness in the multipath fading environment.

[0013] The measured data shows that after adopting this method, the data integrity of the system is improved by more than 90% under typical indoor multipath conditions, and the signal-to-noise ratio (SNR) is increased by 3 - 5dB, verifying the ability of this scheme to suppress multipath interference and the stable data recovery performance in a complex wireless environment. Brief Description of the Drawings

[0014] Figure 1 It is a flowchart of the method of the present invention. Detailed Embodiments

[0015] The following further explains the present invention with reference to the drawings.

[0016] As Figure 1 shown, a method for suppressing the multipath effect of 2.4GHz radio frequency identification, the processing flow of the transmitting end is as follows: Step 1: Analog signal digitization and data preprocessing 1. Analog-to-digital conversion (ADC) and quantization: The input analog signal is sampled periodically according to the Nyquist theorem, that is, the sampling rate ≥ 2 times the signal bandwidth, and the continuous signal is converted into discrete samples. Let the maximum value of the discrete samples be V max , and the minimum value be V min , then the quantization range is [V min , V max ; The quantization range is divided into 8 quantization levels, and according to the quantization step Δ = (V max - V min) / 8, divide each discrete sample by the quantization step size Δ and round up to get a discrete value; map each discrete value to a 3-bit binary code, 000 to 111, and complete the digitization of PCM (Pulse Code Modulation).

[0017] 2. Data Blocking and Modulation Mapping: The digitized bit stream is channel-coded (e.g., convolutional coding) to enhance error robustness. It is then divided into blocks based on the number of subcarriers N (e.g., N = 64), with any missing subcarriers padded with zeros. Quadrature Amplitude Modulation (QAM) is applied to each block of data, mapping the coded bits into modulation symbols on the complex plane and assigning them to N orthogonal subcarriers. The number of subcarriers N is typically determined by the available system bandwidth.

[0018] Step 2: OFDM symbol generation and frame structure construction 1. Frequency domain to time domain conversion: An N-point IFFT transform is performed on the frequency-domain symbols of N subcarriers to generate time-domain OFDM symbols of length N.

[0019] 2. Cyclic prefix (CP) and frame header addition: After extracting OFDM symbols N cp Sample points as cyclic prefix, N cp ≥ Maximum multipath delay / sampling period, spliced to the front end of the symbol to form a complete OFDM symbol. The length of the OFDM symbol is (N+N cp A 4-byte synchronization header, such as 0xABCD1234, is added to the beginning of the frame for frame synchronization on the receiving end.

[0020] Step 3: Digital Signal Serialization and Digital-to-Analog Conversion 1. The OFDM symbols with CP and synchronization header are sequentially spliced into a serial data stream. After being processed by the parallel-to-serial (P / S) circuit, it is converted into an analog baseband signal by the DAC. The sample hold circuit latches the signal amplitude according to the sampling clock.

[0021] Step 4: RF modulation and power amplification 1. Up-convert to 2.4GHz frequency band: The analog baseband signal is mixed with the 2.4GHz local oscillator signal, and the harmonics are filtered out by a bandpass filter (BPF) to complete the RF modulation.

[0022] 2. Power amplification: The signal is amplified by a Class AB power amplifier (PA) to a rated transmit power, such as 100mW, and then radiated from the antenna to the channel. The linearity specification of a Class AB power amplifier is ACLR ≤ -30dBc.

[0023] The processing flow at the receiving end is as follows: Step 1: Radio frequency reception and digital sampling 1. Radio frequency front-end processing: The received signal is amplified by a low-noise amplifier (LNA), down-converted to the baseband by mixing with the local oscillator, the noise is filtered out by an anti-aliasing filter, and then converted into a digital signal by an analog-to-digital converter at the sampling rate of the transmitting end for ADC conversion.

[0024] Step 2: OFDM symbol deframing and frequency-domain conversion 1. Frame synchronization and cyclic prefix removal: Frame alignment is achieved by detecting the synchronization header, each OFDM symbol is segmented, and the CP part with a length of N cp is discarded, and the N-point valid symbol is retained.

[0025] 2. FFT transformation and subcarrier separation: Perform an N-point FFT transformation on the valid symbol, restore the time-domain signal to the frequency-domain symbol of N subcarriers, and separate the data of each subcarrier according to the index.

[0026] Step 3: Signal demodulation and data recovery 1. QAM demodulation and channel decoding: Perform QAM demodulation on each subcarrier symbol, restore the encoded bit stream, and complete channel decoding through Viterbi decoding (corresponding to convolutional codes) to correct transmission errors.

[0027] 2. Data merging: Merge the bit streams decoded from each subcarrier in the block sequence, remove the zero-padding bits, and restore the original data sequence.

[0028] Step 4: Link layer confirmation and power consumption control 1. CRC check and response: Perform a cyclic redundancy check (CRC) on the recovered data. If the check passes: return an ACK response to the transmitting end, and both parties enter the low-power standby mode; if the check fails: return a NACK (Negative Acknowledgement) instruction to trigger the retransmission mechanism of the transmitting end.

[0029] By this method, the influence of the multipath effect on the 2.4GHz signal can be effectively improved, the integrity of the RFID transmission signal can be enhanced, and the communication quality can be ensured.

[0030] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for suppressing multipath effects of 2.4GHz radio frequency identification, characterized in that, It includes a transmitting - end processing flow and a receiving - end processing flow; The transmitting - end processing flow specifically includes: Step 1: Sequentially sample, quantize, and encode the input analog signal to complete pulse - code modulation, converting the analog signal into a digital signal; after channel - coding the digital bit - stream, divide it into blocks according to the number of sub - carriers N, padding with zeros if insufficient; then perform quadrature amplitude modulation on each block of data, map it to complex - plane modulation symbols, and allocate them to N orthogonal sub - carriers; Step 2: Perform an N-point IFFT transformation on the N subcarrier frequency-domain symbols to generate time-domain OFDM symbols, extract the last N cp samples of the symbols as the cyclic prefix and splice them to the front end to form a complete OFDM symbol, and add a 4-byte synchronization header at the beginning of the frame; Step 3: Concatenate the OFDM symbol with a cyclic prefix and the synchronization header into a serial data stream, after parallel - to - serial processing, convert it into an analog base - band signal by a digital - to - analog converter; Step 4: Mix the analog base - band signal with a 2.4GHz local oscillator for up - conversion, filter out harmonics through a band - pass filter, amplify it to the rated power using a power amplifier, and radiate it through an antenna; The receiving - end processing flow specifically includes: Step 1: Amplify the received signal through a low - noise amplifier, mix it with the local oscillator for down - conversion to the base - band, after anti - aliasing filtering, convert it into a digital signal by an analog - to - digital converter according to the transmitting - end sampling rate; Step 2: Achieve frame alignment by detecting the synchronization header, split the OFDM symbol and discard the cyclic prefix, perform an N - point FFT transform on the valid symbol, restore the frequency - domain symbol, and separate the data of each sub - carrier; Step 3: Perform QAM demodulation on each sub - carrier symbol that matches the transmitting - end to restore the coded bit - stream, after decoding to correct error codes, merge the data in the block sequence and remove the padded zero bits to restore the original sequence; Step 4: Perform CRC check on the restored data, return ACK if the check passes, and return NACK to trigger the transmitting - end to re - transmit if it fails.

2. The multipath effect suppression method for 2.4GHz radio frequency identification according to claim 1, characterized in that, In step 1 of the transmitting - end processing flow, the number of sub - carriers N is determined by the available bandwidth of the system.

3. The multipath effect suppression method for 2.4GHz radio frequency identification according to claim 1, characterized in that, In step 2 of the transmission end processing flow, the cyclic prefix length N cp satisfies: N cp ≥ maximum multipath delay / sampling period.

4. The multipath effect suppression method for 2.4GHz radio frequency identification according to claim 1, characterized in that In step 4 of the transmitting - end processing flow, class - AB power amplifiers are used, and the linearity index ACLR ≤ - 30dBc.

5. The multipath effect suppression method for 2.4GHz radio frequency identification according to claim 1, characterized in that, In step 1 of the transmitting - end processing flow, the channel coding is implemented by a convolutional - code encoder; in step 3 of the receiving - end processing flow, Viterbi decoding is used to complete channel decoding.

6. The multipath effect suppression method for 2.4GHz radio frequency identification according to claim 1, characterized in that, In step 4 of the receiving - end processing flow, after returning ACK when the check passes, enter the low - power standby mode.

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